WHITE FIBRES AND GELATINOUS FIBRES. 237 



cells or vesicles. The fibres exist everywhere in the nervous 

 cords, and also make up the greater share of the nervous cen- 

 tres, that is, of the cerebro-spinal centres ; the cells or vesicles, 

 on the contrary, are met with in the cerebro-spinal centre and 

 in the ganglia, and do not exist in the nerves properly so called, 

 unless it be at their peripheral expansions in some of the 

 organs of special sense. It is in the grey matter of the brain, 

 spinal cord, and ganglia, that the vesicular structure is found, 

 the grey matter being made up of vesicles intermixed in many 

 parts with fibres and with a variable proportion of granular 

 or amorphous material. 



The fibres are of two kinds, the tubular or white and the 

 gelatinous or grey, the former being by far the most abundant. 

 The gelatinous or grey fibres are found principally in the sym- 

 pathetic nerve, yet exist also in many of the cerebro-spinal 

 nerves. 



The white fibres consist of a fine membranous tube, enclos- 

 ing a peculiar soft substance, while this contained substance is 

 distinguishable into a central part placed like a sort of axis in 

 the middle of the tube, and a peripheral portion surrounding 

 the axis, so as to occupy the space between the axis and the 

 tubular enclosing membrane. Many of the tubular nerve- 

 fibres, under the microscope, exhibit a varicose appearance 

 that is, they seem to be dilated or swollen out at short dis- 

 tances along their length, and contracted in the intervals be- 

 tween the dilated parts. Such appearances occur in the brain 

 and spinal cord, and in the intracranial portions of the olfactory, 

 the ophthalmic, and acoustic nerves. It appears, however, that 

 these fibres are cylindrical like the other nerve-fibres, and 

 that they only take on this appearance when handled, and 

 thereby subjected to pressure. This effect is most apt to occur 

 in fibres of small size, as in those ranging from the 1200th 

 to the 3600th of an inch in diameter. 



